In many cases it is advantageous that telecommunication equipment is modular so that the telecommunication equipment comprises a rack and telecommunication devices are installed directly or with the aid of device-frames in the rack. In this document, the term “telecommunication” includes also “data communication” and other data and message transfer technologies. The above-mentioned telecommunication devices may constitute, for example but not necessarily, one or more internet protocol “IP” routers, one or more Ethernet switches, one or more Asynchronous Transfer Mode “ATM” switches, one or more MultiProtocol Label Switching “MPLS” switches, and/or one or more packet optical switches.
A rack of modular telecommunication equipment may comprise for example mutually parallel rails that are substantially vertical when the rack is in its operating position. The telecommunication devices of the telecommunication equipment can be for example plug-in units that are installed in plug-in unit places of one or more device-frames that are, in turn, attached to the vertical rails of the rack. In many cases, the lower part of each device-frame comprises an air-intake for receiving cooling air and the upper part of the device-frame comprises an air-outlet through which the cooling air can leave the device-frame. When the cooling air flows from the air-intake to the air-outlet, the cooling air transfers heat from the plug-in units that are installed in the plug-in unit places of the device-frame under consideration. In many cases, the device-frame further comprises a blower for moving the cooling air through the device-frame. Furthermore, the device-frame may comprise a filter for filtering the cooling air prior to the cooling air meets the plug-in units.
An inherent inconvenience related to device-frames of the kind described above is that the cooling arrangement has to be designed according to the most demanding circumstances. Thus, for example the above-mentioned air-intake that has been designed according to the most demanding circumstances may require unnecessarily much room in the vertical direction in less demanding circumstances. The space requirement in the vertical direction is critical because, in many cases, it determines the number of device-frames that can be installed in a rack.